1. Field of the Invention
This invention relates to a laser beam recording apparatus for recording an image on a photosensitive material by scanning the photosensitive material with a laser beam modulated in accordance with an image signal.
2. Description of the Prior Art
Light beam scanning recording apparatuses, wherein a photosensitive material is scanned with a modulated light beam and an image is thereby recorded on the photosensitive material, have heretofore been used widely. As one of the means for producing a light beam in the light beam scanning recording apparatuses, a semiconductor laser has heretofore been used. A semiconductor laser has various advantages over a gas laser, or the like, in that the semiconductor laser is small in size, cheap and consumes little power, and in that the laser beam can be modulated directly by the drive current applied thereto.
FIG. 2 is a graph showing the optical output characteristics of an ordinary semiconductor laser with respect to the drive current. With reference to FIG. 2, the optical output characteristics of the semiconductor laser with respect to the drive current change sharply between a natural light emission region (LED region ) and a laser oscillation region. Therefore, it is not always possible to apply the semiconductor laser to an operation for recording a continuous tone image. Specifically, in cases where intensity modulation is carried out by utilizing only the laser oscillation region, in which the optical output characteristics of the semiconductor laser with respect to the drive current are linear, it is possible to obtain a dynamic range of the optical output of only approximately 2 orders of ten at the most. As is well known, with a dynamic range of this order, it is impossible to obtain a continuous tone image having a high quality.
Accordingly, as disclosed in, for example, U.S. Pat. No. 4,849,980, it has been proposed to obtain a wide dynamic range of the optical output by controlling the drive current for a semiconductor laser over the natural light emission region and the laser oscillation region and thereby carrying out optical intensity modulation.
In general, the wavelength of a laser beam produced by a semiconductor laser fluctuates due to a change in the ambient temperature, or the like. Such a phenomenon is referred to as "mode hopping." If the mode hopping phenomenon occurs in a semiconductor laser, which is utilized as a source for producing recording light in a laser beam recording apparatus, the optical output of the semiconductor laser will fluctuate, and the image density of a recorded image will become non-uniform. Such a problem occurs due to various factors in cases where an automatic power control circuit (hereinafter referred to as an APC circuit) is provided. With the APC circuit, the intensity of a laser beam radiated rearwardly from the semiconductor laser (i.e., a laser beam radiated in a direction reverse to the direction along which the forwardly radiated laser beam to be used for the recording of an image is produced) is detected. Alternatively, the intensity of part of the forwardly radiated laser beam which has been separated from the main forwardly radiated laser beam by a beam splitter, or the like, and which serves as a monitor laser beam is detected. The drive current applied to the semiconductor laser is controlled such that the detected intensity may be kept at a predetermined value.
For example, if the mode hopping phenomenon occurs in cases where the rearwardly radiated laser beam is monitored, the ratio of the intensity of the rearwardly radiated laser beam to the intensity of the forwardly radiated laser beam will change. Therefore, in such cases, even if the drive current applied to the semiconductor laser is controlled such that the rearwardly radiated laser beam may become stable, the intensity of the forwardly radiated laser beam to be used for the recording of an image will vary before the mode hopping phenomenon occurred and after the mode hopping phenomenon occurred. Also, part of the monitor laser beam is reflected repeatedly between opposite surfaces of a cover glass of the photodetector of the APC circuit, and interference occurs between the repeatedly reflected monitor laser beam and the monitor laser beam which directly travels to the light receiving element without being reflected repeatedly. The state of the interference changes due to fluctuations in the wavelength of the laser beam which are caused by the mode hopping phenomenon. As a result, the amount of the laser beam received by the photodetector changes. In such cases, the intensity of the forwardly radiated laser beam fluctuates.